This invention relates to a radiation image storage panel having a stimulable phosphor layer. More particularly, it relates to a radiation image storage panel which is protected from deterioration due to the penetration of water, which endures for a long period of use, and which is high in sharpness.
Radiation images like X-ray images are often used in diagnosis of diseases.
In order to obtain the X-ray images, X-ray image converting methods of directly taking out images from phosphors have been devised in place of light-sensitive silver halide materials.
The methods include a method in which the radiation (generally X-ray) transmitted through a subject is absorbed to a phosphor, and thereafter this phosphor is excited by light or heat energy to bring the radiation energy stored by above absorption to radiate as fluorescence, which fluorescence is detected and formed into an image.
Specifically, U.S. Pat. No. 3,859,527 and Japanese Provisional Patent Publication No. 12144/1980 disclose radiation image storage methods in which a stimulable phosphor is used and visible light or infrared rays are used as stimulating light.
This method employs a radiation image storage panel (hereinafter often referred to as "storage panel") comprising a support formed thereon with a stimulable phosphor layer (hereinafter often referred to simply as "stimulable layer"), wherein radiation transmitted through a subject is applied to the stimulable layer to store radiation energy corresponding to the radiation transmission degree of all areas of the subject to form an image, and thereafter this stimulable layer is scanned with the stimulating light to bring the radiation energy stored in the areas to radiate to convert this into light, thus obtaining an image according to signals based on the strength of this light.
This image finally obtained may be reproduced as a hard copy, or may be reproduced on a CRT.
The storage panel to be used in this radiation image storage method irradiates stored energy by scanning of the stimulating light after stored radiation image information so that stored radiation images can be again carried out after scanning, the storage panel can thus be used repeatedly.
The above storage panel shall desirably endure for a long time or many times of repeated use without deteriorating image quality of the obtained radiation images. In order to satisfy the requirement, it is requested that the stimulable layer of the above storage panel should be sufficiently protected from outer physical or chemical excitation.
In the conventional storage panels, there have been employed the method in which a protective layer to cover a surface of the stimulable layer on a support of the storage panel in order to solve the above problems. This protective layer is, as disclosed in, for example, Japanese Provisional Patent Publication No. 42500/1984, formed by directly coating a coating solution for the protective layer on a stimulable layer or formed by the method in which a previously formed protective layer which had been prepared separately is adhered on a stimulable layer.
As the protective layer, a thin protective layer comprising an organic polymer has generally been used. The reason why such a thin protective layer has been used is to prevent lowering of sharpness of a storage panel as hard as possible.
A relationship between sharpness and a thickness of a protective layer of a storage panel having a stimulable layer is shown in Table 1 by using MTF (Modulation Transfer Function) of a spatial frequency of 1 lp/mm and 2 lp/mm. In Table 1, PET is a polyethylene terephthalate film.
TABLE 1 ______________________________________ Protective Thickness 1 lp/mm 2 lp/mm layer (.mu.m) (%) (%) ______________________________________ None -- 78 41 PET 8.5 74 39 PET 11 64 35 PET 25 59 29 PET 70 54 26 PET 175 48 24 Glass 550 43 20 ______________________________________
As shown in Table 1, when the protective layer is thicker, sharpness becomes low. A reason that may be mentioned is that reflected scattering light of incident stimulating light at a surface of a stimulable layer is reflected at an interface between the protective layer and air and is then re-incident on the stimulable layer. When the protective layer is thicker, the reflected scattering light extends further and the result is image information of images other than the images to be detected so that sharpness thereof is lowered.
In a conventional film/screen system to be used for X-ray photographing, since the MTF in the case of 1 lp/mm is about 65% and in the case of 2 lp/mm is about 35%, it is not preferred, in a storage panel, to inferior to the values of the above film/screen system. Accordingly, in the protective layer of the conventional storage panel an organic polymer film having a thickness of about 10 .mu.m or so is used.
However, this basic problem of decreasing in sharpness due to providing a protective layer has not yet bear solved. It has been strongly desired to make a storage panel with higher and higher sharpness.
Also, the thin protective layer comprising an organic polymer which is conventionally used is transmissive to some extent of water and/or moisture. Thus, the stimulable layer absorbs water so that the storage panel's sensitivity to radiation is lowered. Further, deterioration of stored energy between after irradiation of radiation and before accepting irradiation of stimulating light fading is large, and thus fluctuation and/or deterioration of image qualities of the obtained radiation images is/are caused.
For example, moisture permeability coefficient of PET having a thickness of 10 .mu.m is about 60 g/m.sup.2.24 hr and thus at most 60 g of moisture per unit area transmits therethrough in a day. Even in an oriented polypropylene (hereinafter abbreviated to "OPP"), the moisture permeability coefficient level is about 15 g/m.sup.2.24 hr.
Further, in the conventional storage panel having the above-mentioned thin protective layer, since surface hardness of the protective layer is low, flaws on the surface of the protective layer due to contact with a machine component such as a conveying roller, etc. at transmission. Or, flaws will likely be developed due to a crack or a fold in the stimulating layer since impact resistance is insufficient in a thin protective layer. Therefore, there is a problem that image qualities of the obtained radiation image become worse with repeated use.
On the other hand, when the protective layer is thick, the defects caused by a thin film can be overcome but again sharpness becomes low as mentioned above. It has been desired, over two contrary phenomenon, to improve from the aspects of moisture resistance, strength and impact resistance without impairing sharpness.
One such attempted storage panel can be prepared by sealing at edge portions of a laminating product comprising a support, a stimulable phosphor and a protective layer in order to improve humidity resistance. As the method of sealing, there have been employed, for example, the method in which edge portions are sealed with a sealing agent and the sealing agent is solidified by a solidifying agent from outside (Japanese provisional Patent Publication No. 237099/1986) or the method in which edge portions are sealed at the state of being coated with elongated portions of the protective layer (Japanese provisional Patent Publication No. 237100/1986).
However, by sealing them only by the above methods, the effect of water to permeating a protective layer to a stimulable layer cannot sufficiently be prevented.
As described above, in a storage panel which employs the conventional stimulable phosphor, there is the disadvantage that sharpness becomes necessarily low by providing a protective layer.
Also, in the conventional storage panel, when a thin protective layer is employed in order to prevent lowering in sharpness of images, there are problems that fluctuation or deterioration of obtained radiation images are caused by chemical stimulations from outside, particularly by permeation of water or moisture, or breakage of the stimulable layer, etc. is caused by physical stimulations from outside. On the other hand, for the purpose of protecting from such chemical and physical stimulations, when a thick protective layer is used, there is a problem that sharpness of images becomes low.
Further, when preparing the conventional storage panel, adhesion between a protective layer and a stimulable layer and/or the stimulable layer and a support may be sometimes carried out by using an adhesive agent. There is the problem in that heat added to heat and cure the adhesive agent, causes the storage panel to deform or warp, and as a result, damages are caused in the stimulable layer whereby the quality of images becomes low.
Moreover, even when a protective layer is provided on a stimulable layer and edges of a storage panel are sealed, a stimulable phosphor absorbs moisture during preparation of a storage panel so that the initial characteristics of the storage panel becomes low. Furthermore, the fluctuation of characteristics of each storage panel is a problem since the extent of the above moisture absorption depends largely on atmosphere at the preparation step. Thus, when heating and drying are carried out after sealing edges of the storage panel, there is the problem that air at the inside of the storage panel expands whereby it becomes a cause of destruction or deformation of the storage panel, and therefore it is not preferred.